Filtering electric currents



May12,1931. G. B. CROUSE 1,804,859

FILTERING ELECTRIC cURRENTs Filed Deo. 12, 1923 2 sheets-sheet 1A /3 LD a2 2f l l l A May 12, 1931. G, B, CR'OUSE 1,804,359

I FILTERING ELECTRIC CURRENTS Filed Dec. 12l 1923 2 Sheets-Sheet 2 4 ATT E Patented May l2, 1931 UNITED STATES PATENT OFFICE GEORGE B. CROUSE, OF STANFORD, CONNECTICUT, ASSIGNOR, BY MESNE ASSIGNMENTS, TO CONNER CROUSE CORPORATION, F NEW YORK, N. Y., CORPORATION OF NEW YORK FILTERING ELECTRIC CUR-RENTS Application filed December 12, 1923. Serial No. 680,096.

This invention relates to filtering electric currents, and more particularly to the separation from a direct current supply of all alternating current components or ripples.

There are many devices which require for their satisfactory operation a. source of direct current which is absolutely free from all alternating current components or short period variations. l

A device of this character in which the requirements are particularly severe, is the modern radio receiver, employin audion tubes as detectors and amplifiers. ot only are two such sources of electrical energy re,- quired for the operation of the audion (i. e., one for heating the hot cathode and one for supplying the thermionic current) but any short period variation of the supply, which might not be objectionable in a single tube, is amplified many hundreds of times if several tubes are employed in cascade. There are also other difliculties which will be mentioned later.

He'retofore it has been the practice to supply theypower for operating theaudions in a radio receiver from prim ary o1' secondary batteries, as being the only practical means for obtaining electrical energy of the requisite smoothness. The disadvantages of a battery for this purpose are many. If secondary batteries are employed, the strongly corrosive electrolytes required cause damage to furniture and floor coverings, and the necessity of continually recharging is troublesome, in addition to which the life of the battery is comparatively short. If dry batteries are employed, their life is short, and the expense of maintenance is'high.

It has therefore been obvious to inventors 4 for some time that some means of operating a radio receiver directly from the usual A. C. lighting mains would be very desirable, and numerous attempts have been made to accomplish this purpose. For instance, in Patent No. 1,432,022, granted. on October 17, 1922 to R. A..Heising, means for operating the filaments'of audions on alternati current is disclosed. In this method, the aments are lighted from alternating current without rectification, and the grid is connected to the middle point of the filament, either directly or through an external potentiometer. I have found that this method is unsatisfactory for a radio receiver, due to the fact that the amplifying characteristics of the audion are diilerent at different potentials between the grid and filament. Therefore that half of the filament which at any moment is negative with respect to the grid is not exactly balanced by the positive half, with resulting noise in the output of the receiver and troublesome modulation of the received signal.

It has been proposed to rectify the 'alternating'current, and-then to smooth out the resulting fluctuation by means of electrical wave filters, of the general type disclosed by G. A. Campbell in Patent No. 1,227 ,113, granted on May 22, 1917. I have found that l'ilters. of this type, to be even reasonably satisfactory, require impractically large inductances and condensers.

Still another method which has been proposed is that shown in Patent No. 1,411,814

granted to`H. M. Stoller, on April 4, 1922. This method employs a-motor generator, for generating direct currents of the requisite voltages. It has long been known that a generator is not a satisfactory means of supplying energy to an audion, due to the ripples caused by the commutation, and to remedy this, Stoller uses a modified form of wave filter, in which only inductances and resistances are employed. This method is reasonably satisfactory for the purpose for which it was designed, which was to receive-spark signals on an aeroplane. However, the resulting noise and'lnodulation with this system, make it totally unsuitable for receiving very weak signals, or for radio telephone reception.

An object of this invention is to provide economical and eicient apparatus for the substantially complete suppression of all alternating components present in a ,source of rectified current. A further object is to provide an electrical network for use between anaudion circuit and a direct current source containing alternating components, which 'network permits the transmission of direct current but offers a substantial impedance to all. alternating components and a maximuni impedance at the frequency of the most disturbing alternating Acomponent of the source.

More specifically an object is to provide an electrical network of the type stated, 'and in which the impedance elements are arranged as an alternating current Wheatstone bridge across which the source of current and the audion circuit are connected as conjugate In the following specification, I have described. oneembodiment of my invention, in

fpart of this specification,

Fig. 1.

1 direct current `is caused to iiow through aV portion of each of the paths and the load circuit.

While in the following disclosure, I have `describeda particular application of the invention to a radio receiving set employing audion tubes, as such an application repre-` sents the `most severe requirements with which I ain familiar, the invention is not limited to this use, but is equally applicable to any case requiring a source of direct current `free from ripples, or short period disturbances.

Referring to the drawings which form a l Fig. 1 is .an electrical diagramshowing the general principles of the invention,

Fi g. 2 shows the potential 'characteristics obtaining at various points of the circuit of Fig. 371s an electrical diagram showing a Vmodification of the invention suitable for' large currents.

Fig. 4 is an electrical diagram showing v the invention applied to a radio receiver.

' turns and connected kin. series Fig. 5 represents in simplified form, some of the elements of Fig. 1.

,s 6`is a graphic illustration of the suppression characteristics of the filteriietwork. The general principle of the invention may'best be undfn'stood` by reference to Fig. 1'. In this figure, I have shown the direct current supply-as derived from therectif'ication of alternating current, as for instance GQ cycle current as'ordinarily, supplied for light and power. land 2 are the terminals which areconnected ftp such a source of supply. l3 isthe primary winding of a transformer hav- 'j ing an iron core ,4, and two secondary Wind-4 ings, 5 and 6, having an equal numberof as shown. 7 and 8 represent twoiectifiers, which may be of any type suitable toit/he' voltage and current requirements of the load demand. The outer terminals of the rectifiers are connected together to the wire 9, to Vforni one side of the rectified supply, the other side being obtained by a connection 10 tothe inside connection of the secondaries, 5 and 6. this arrangement alone, there will be delivered to the conductors 9 and-10 a pulsating undirectional potential, as shown at B, Fig. 2, Ain this figure sdiowing the voltage applied at the input terminals 1 and 2, Fig.

Now by the addition of an electrostatic condenser 11, coniieted across the conductors 9 and A10, the voltage characteristic ai-'aiiable for the load circuit is changed to approximatelyy that shown at C, Fig. 2 due to the storing action of the condenser. 'Such With l a voltage curve contains a direct current coiii- Ponent, upon which is superimposed an al. iciiiating voltage' containing iii general a large number of different frequencies, the lowest frequency being twice the frequency of the primary supply voltage, provided that the rectifiers 7 and 8 have equal icctifyiiig properties under load. I

Such a voltage supply is totally unsuitable for application to such devices Aas audion tubes for-either transmitting or receiving radio`telegraph or telephone messages. In

(order to separate all of these alternating current disturbances from the direct current component, I cause the current to flow through two parallel path .0ne path com prisiig the condenser 12 an 4-tlie inductances 14 and 15 and the other path comprising the inductances 16 and 17 'and' the condenser-13, as shown in simplified form in Fig. 5. In

esv

practice these four inductances. are prefei-,

ably all wound on Vthe same iron core 18 and in such a y tances reinforce their self inductances.

It will be apparent from ai consideration of this diagram, that the effect'ofrthe-con--f densers 1 2 and 13' will be to voffset a portion of the inductance of the coils1'4 and 17 respectively, and that therefore, if "We'suitably increase the inductance' ofthese two coils over that ofthe coils 1,5 and 16, we may obtain at the junction points 19 and 20, equal A and opposite potentials for any particular frequency.- In practice, I prefer to design direction that their mutual inducy lio the circuit to obtain equal potentials at 'a'.vQ-v' frequency Yof twice the supply frequency. By employing comparatively large values of theconden'sers 12and 1,3,.this equality of potential will -not only be obtained, for the double frequency component. but to a large extent also for all higher frequencies, leav ,ing only a very small percentage of these higher, equenciesbetween the points 19t and-2.0. i

y In order to illustrate `this numerical values may be. assigned to the constants. of the circuit. Suppose that-the `coilsqland-16, are

. given by the formula is approximately 7,500 ohms at f: 120 cycles. Assume the condensers 12 and 13 to have a value of microfarads each. Their reactauce at 120 cycles, given by the formula 1 Xfarfo Will be approximately lohms. Then in order to obtain equal potentials at this frequency, it will be necessary to make the inductive reactance of the coils 14 and 1T, 7.500 ohms-H250 ohms=7.630 ohms.

As the frequency is increased, the reactancel of the condcnscrs becomes smaller. and the rem-tance of the inductances larger, thereby producing unequal potentials at the junction points for these higher frequencies. For instance, at 240 cycles, the reactance of the condensers Would be decreased o r (ohms, the reactances of the inductances being doubled, or, for coils 15 and 16, 15,000

ohms and for coils 14 and 17, 15,260. Subtracting the condensive reactance of ohms from the latter value, gives values of the arms 14 and 17 of 15,195 ohms. Therefore at a frequency ot' 240 cycles, the potential difference will be reduced to about 3% of its total value.

'lhe suppression characteristic of the filter may be plotted by computing the inipedances of the bridge arms at other frequencies, and it will be found that the curve has the form illustrated in full'line in Fig. 6. The attenuation at the frequency of the maximum alternating component present in the rectified current, i. e.. cycles, will be infinite as the bridge is balanced for that frequency. The attenuation at other frequencies is, of course,

less but is relatively high at'all other fre-- quencies presentin the rectified current. The magnitudey of such other frequencies is of an order materially lower than that of the fundamental frequency and the attenuation of the bridge circuit may effect the required suppression of such components. Where more complete elimination is desired, the attenuation characteristic may be raised by additional impedances associated with the audion circuit.

Returning again to Fig. 1, the load is connected across the points 19 and 20, the load being represented by the resistance 21. It will be apparent that all direct current will be obliged to 4llow across the load 21, due to the position of the condensers 12 and 13 inthe vparallel circuits, which causes the bridge to be unbalanced for direct current. In order to absorb the small remaining percentage of the higher frequencies which are not balanced out, I employ an inductance 22 in series with the load, and a condenser 23 in parallel with the load.

The suppression characteristic of the in` ductance '22 and condenser 23 has the general form of the dotted line curve of Fig. 6. The degree of suppression depends upon the values of these impedances and the load but the general characteristic of an arrangement of this type is to offer progressively higher suppression for increasing frequencies.

, In order to give a clearer idea of the magnitude of the various electrical quantities which I have found to be satisfactory in practice for carrying out my invention, I will describe an actual circuit for supplying the plate current to a radio receiving set, which required volts and 50 millianiperes.

In the construction of this circuit, I cmployed secondaries 5 and 6 giving 300 volts apiece, the rectifiers 7 and 8 being of the two element bot cathode type. Condenser' ll had a capacity ot' 2 inicrofarads. Condensers 12 and 13 were of the electrolytic type, with rated capacities of 28 microfarads each. The coils 14, 15, 16 andv17, had effective inductances (including their mutual inductances) of approximately 10 henries each. Coil 22 had about 6l1enries inductance, and condenser 23 was of 1 microfarad capacity. In the particular case described, I have found that the values given above may be varied through very Wide limits, Without affecting the effective operation of the device, and it Will therefore be understood that the invention is in no Way limited to the proportions given. The proper proportion of the elements of the circuits to lit any given load demand will be apparent to anyone skilled in the art.

It is obvious that my invention as described above may be applied to smooth out the commutator ripples, when using a direct current generator as a source of supply, by merely connecting the terminals of the machine to conductors 9 and 10, in place of the transformer and rectiliers shown in Fig. 1.

In. those applications Where it is necessary to supply comparatively large currents at relatively low voltages, it is obvious that the large direct current flowing through the windings 15 and 16 sets up a constant 'flux in the core 18, which due to the partial saturation of the iron, reduces the effective inductance of the coils. In order to remedy this condition, and to reducethe size and Weight of the iron core, I employ a modification as shown in Fig. 3.

In this figure, I have shown the same circuit as that of Fig. 1, withthe'corresponding parts identically numbered. The additional element is the coil 24, which is connected directly. across the leads from the rectifier and wound on the core 18, in such a manner that the direct current which will be shunted through the coil will be in the trl) Aas

opposite direction to the current iiowing in the coils 15 and 16. In the coil 24 I employ ka comparativelly large number of turns, for n the first place, I obtain the,

two reasons. 0 requisite number of de-magnetizing ampere turns,- without the dissipation of a ,large amount of current. In the second' place, it

will be noted that whereas the direct current Aflowing in this auxiliary coil is usefully demagnetizing, the alternating current is also opposed to the alternating current flowing in the coils 14 15, 16 and`17 and therefore tends.-

to reduce the inductance of these coils, which is detrimental. By the use of a large nurnber of turns in this auxiliary coil, in` comparison with the number of turns of the main coils, the alternating currentiiowing in the auxiliary coil will be reduced faster than the number of turns of the coil is increased, so that the detrimental effect nienvtioned is minimized.

' As mentioned above, the supply of electricalpower to the audions of a radio receiver is a most severe requirement, since any disturbance of audio frequency is amplified many hundreds, or even thousands of timesu ap lied in a similar manner to any receiver.

n Fig. 4, 25 is an antenna, 26 an inductance coil suitable for radio frequencies, 27 a variable condenser and 28a ground connection. The antenna circuit is tuned by means of the condenser 27, to the frequency of the incoming radio signal wave in the usual manner. Two stages'of radio frequency amplification are shown, with audion tubes 29 and 30. Tubes 29 and 30 are coupled by means of the radio frequency transformer 31 while tube 30 is coupled to the detector tube 33 by the transformer 32. The usual grid leak and condenser 34 are employed in the grid circuit of the detector tube. Small condensers (say l.002 in. f. ca acity) 35 and 36 are connected between the lament connection and the outer end of the transformer primaries, for the purpose of 'by-passing the radio frequency currents and thus prevent the coupling which might occur between tubes, if this radio frequency current were obliged to traverse the entire plate circuit.

Detector tube 33 is coupled to the first audio vtube 37 by means of the iron core transformer 38, and.- in like manner, tube 37 is 2cou led to the vsecond audio tube 39 by means o the transformer 40. A suitable telephone device 41 is placed in the plate circuit of the last tube. 1

The above description of the radio setpis merely given to make the diagram clear, since circuits of this type are well known to those familiar with the art.

' Turning now to the power supply circuit, this circuit up to the terminals 42 and 43 1s identical with that shown in Fig. l, the corresponding parts are identically numbered and the manner in which current is supplied to the audions is obvious from the preceding description.

It will be noted from Fig. .4 that I prefer to place all of the audion filaments in' series. 80

This is not essential to the application of my invention, but it hasthe advantage that pnly one set of rectiiers and filters 1s required,

which usually offsets the disadvantage of the l lifghtly lower efliciency of the arran ement. 95

sometimes find it desirable, particularly-if it is required to operate the receiver with a head set instead of a loud speaker, to employ an anti-resonant circuit in the plate circuit, consisting of the 'condenser 44 and the inductance 45, the values being designed to -make the circuit anti-resonant at, usually,

twice the frequency of the power supply:

For the purpose of absorbing the addltional voltage in the filament circuit, the resistance 46 is employed. `47 is a resistance for 100 the purpose of regulatin the plate voltage to the detector tube and t e radio frequency tubes, and may be made adjustable if desired. This resistance is shunted by the condenser 48 to prevent intertube coupling and also loss -105 of eiiiciencyin the detector tube plate circuit'. The condenser 49 is employed or the same pur se. .An adjustable resistance 50 is placed around the filament a detector tube. u

Should it be desirable or necessar to bias the grids of any of the amplifier tu es, .it is only .necessary to connect the rid connection to a suitable point on the lament circuit, since the supply is q uite enough-to make the use of a separate biasing battery unnecessary.

Various combinations and modifications of the' above arrangements will readily sug- 120 l gest themselves to those'familiar with the art,I to suit any particular requirements of load conditions. a

I claim 1. A filter network for use between a of rectiiied current and an audion` circuit, said network comprising impedances forming a plurality of e ementary circuits between the input terminals and output terminals of the network, said elementary circuits cooperof the tube 33 t0 Y control its brilliancy, this being vdesirable on 110 ating to pass direct current through the audion circuit and to by-pass alternating components about the audion circuit, two of said elementary circuits each including an inductance, and mutual inductance between said inductances.

2. In apparatus for energizing an audion circuit from a source of rectified current, a

filter having a pair-of input terminals for l, connection to said source, a pair of output terminals for receiving connections from tlieaudion circuit, and an impedance network between said pairs of termina1s,said network comprising circuit elements connecting the respective terminals of said pairs of terminals to provide a series path for supplying 'direct current from said source to said circuit, and means cooperating with said circuit elemeiits of the direct current path to suppress the transmission of alternating currents from said soui'ce to said audion circuit and to effect a maximum suppression at a frequency in the lower range of the alternating components present in the rectified current. v

3. In apparatus for energizing an audion circuit from a source of rectified current, a filter having a pair of input terminals for connection to said source, a pair of output terminals for receiving connections from the audion circuit, and an impedance network betweenl said pairs of terminals, said network comprising circuit elements connectingthe respective terminals of saidpairs of termi nals to provide a series path for supplying direct current from said source to said circuit, and means cooperating with said circuit elements of the direct current path to suppress the transmission of alternating currents from said source to said audion circuit and to effect a maximum su pression at the frequency of the most disturbing alternating component present in the rectified current.

4. In apparatus for energizing an audion circuit from a source of rectified current, a filter having a pair of input terminals for connection to said source, a pair of output terminalsffor receiving connections from the audion circuit, and an impedance network between said pairs of terminals, said network comprising circuit elements connecting the respective terminals of said pairs ofterminals to provide a series path for supplying direct current from said source to said circuit, and means cooperating with said circuit elements of the direct current path to suppress the transmission of alternating currents from said source to said audion circuit and to efect a maximum suppression as the frequency of the maximum alternating component pres- `ent in the rectified current.

5. In apparatus for energizing an audion circuit from af source of rectified current, a filter having a pairv of input terminals for 5 connection to said source, a pair 'of output terminals for receiving connections from the audion circuit, and an impedance network between said pairs of terminals, said network comprising circuit elements connecting the respective terminals of said pairs of terminals to provide a series path for supplying direct current from said source to said circuit, and means cooperating with said circuit elements of the direct current path to suppress the transmission of alternating currents from said sourceto said'audion circuit and to effect a maximum suppression at the frequency of the maximum alternating component present in the rectified current and a high suppression at all harmonics thereof.

6. In apparatus for energizing an audion circuit from a source of rectified current, a filter having a pair of input terminals for connection to said source, a pair of output terminals for receiving connections from the audion circuit, and an impedance network between said pairs of terminals, said network comprising circuit elements connecting the respective terminals of said pairs of terminals to. provide a series path for supplying direct current from said source to said circuit, and means co-operating with said circuit elements of the direct current path to suppress the transmission of alternating lcurrents from said source to said audion circuit and to eiiect a maximum suppression at a predetermined finite frequency and a high suppression at all frequencies present in the rectified current.

7. An apparatus for energizing an audion circuit from a source of alternating current, a full Wave rectifier, a filter having a pair of input terminals for connection to said full wave rectifier, a pair of output terminals for receiving connections from the audion cir` cuit, and an impedance network between said pairs of-terminals, said network comprising circuit elements connecting the respective terminals of saidpairs of terminals to provide a series path for supplying a direct current from said source to said circuit, and means cooperating with said circuit elements of the direct current path to suppress the transniissiou of alternating currents from said source to said'audion circuit and to effeet the maximum suppression at double the ins` j source and is unbalanced for direct current.

10. In apparatus-for energizing an audion circuit from a source of rectified current, a

plurality of impedances serially arranged as the four balancingarms of a Wheatstone bridge having conjugate pairs ofjunction points serving as the terminals for said' audion circuit and said source, respectively, the two alternative paths through said circuit providedby the pairs of non-adjacent arms being one conductive and the other substantially non-conductive or direct current, andthe impedances of the arms being so related that the bridge is substantially balanced for the frequency of the maximum alternating current component of said source. 11. In apparatus for energizing an audion circuit :from a source of rectified current, im-

pedances providing parallel alternating current paths between two input terminals to which said source may be connected, output terminals located at intermediate points in the respective paths for' connection to the audion circuit, the im edances of the four portions into which sai paths are divided by said four terminals being such that the network functions as a Wheatstone bridge sub'- 'stantially balanced at the frequency of the maximum alternating. component of said sourceqcurrent, the bridge arm provided by one of said four portions being non-conductive for direct current.

l12. Apparatus for preventing the alternating components of a source of rectified currentV from traversin an audion circuit comprising a plurality of lmpedances forming an alternatlng current Wheatstone bridge having said source and said circuit arranged as the con- .jugate arms thereof, said impedances being so proportioned that the bridge is substantially balanced at the frequency of the alternating component which causes the greatest disturbance in the audj'on circuit.`

13. In apparatus for lilterin current supplied from a source to a load clrcuit, impedance lelements arranged as a Wheatstone bridge capable of being balanced for alternating currents. two impedance elements comprising non-adjacent balancing arms of said bridge being provided by inductances, the impedanceV elements of the other two nous adjacent arms being lcomposite vand comprising inductance and capacity in series, and a core of magnetic material common to the inductances of all of said balancing arms, the respective conjugate pairs of junction points of said arms serving as terminals for the source and the load circuit, respectively. Y

14:. An electrical filter for use between anV audion circuit and a source of direct current having alternating components, said filter comprising four v impedances serially connected as the four balancing arms of a Wheatstone bridge, two non-adjacent arms being conductive, and'the other two non-adjacent arms being substantially non-conductive or direct curr-ents, the conjugate pairs of junction points of said arms serving as terminals ,for said audion circuit and said source,

respectively.

15. In apparatus for energizing an audion circuit of a radio receiver from a source of direct-currenthaving alternating components, impedances providing parallel alternating current paths between two terminals to which said source may. be connected, eachy of said paths comprising conductive and non-conductive elements in series and the non-conducting elements in the two paths being reversely arranged, and terminals forthe load circuit at intermediate points in the respective paths which are of substantially equal alternating current potential but of unequal direct current potential.

16. Means for supplying the audions of a raido receiver .with rectified current ree from alternating components, comprising a' vcircuit including a plurality of parallel paths and means for connecting the filaments of the audions in series between the paths at points of substantially equal and simultaneous alternating current potential, but of unequal direct current potential. 17. In apparatus for supplying the filament and plate circuits of a plurality of audions from an alternating current supply,

a rectier for converting alternating current j from said supply. into direct current having .alternating components, a filter network energized by said rectifier output and comprising a plurality lof impedances proportioned and related to suppress said alternating components, means for connecting the filaments of the audions in series between points in said network which are at diii'erent direct current but substantially equal alternating current potentials, and meansfor energizing the plate circuits of said audions from said network.

1,8. In apparatus for supplying the lilament and plate circuits of'a plurality of audions from an alternating current supply, a rectier for converting alternating current from said supply into direct current having alternating components, a filter network energized by said rectifier output and comprising awplurality of impedances proportioned and related to suppress said alternating components, said network having outputterias minals across which there eirists a direct current potential of the order required for energizing the plate circuits=and means including a resistance `for connecting the laments of said audions in series across said output terminals.' 19. In a system for supplying the audions of a radio receiver from an alternating current supply, the combination of means for y rectifying the alternating current, a condenser connected across the rectied output circuit, a Wheatstone bridge in parallel with the` condenser' and substantially balanced l for alternating current but unbalanced for direct current, means` for connecting the audion circuits between the points of substantially equal alternating current potential but of unequal directl current potential whereby only direct current free froml short period variations will flow therethrough, an inductance in series with said connecting audion circuits 'and a condenser in parallel With the means. Y 20. AIn apparatus for filtering direct current containing alternating components to obtain smooth direct current, an iron core, a plurality of coils wound on the core and constituting the arms respectively of a Wheatstone bridge and condensers 1n series with the coils of one paironly of opposite larms of the bridge. v

21. In apparatus for filtering directeurrent containing alternating components to obtain smooth direct current for a load circuit, a common iron core, aplurality of coils wound on the core, a plurality of condensers, said coils and condensers being connected alternatively in series, spaced points in said series-connected impedances serving as in- 40 put terminals for the apparatus, and means for connecting the load circuit to the coils at points of substantially equal lpotentials for the alternating components, but of unequal v potentials for the direct current. In testimony whereof I aix my signature.

n GEORGE B. CROUSE. 

